When forming an internal thread in the inner surface of a starting hole formed in a workpiece by cutting the inner surface of the starting hole with a tap, chips may get stuck between the workpiece and the tap, thereby damaging the tap or resulting in poor accuracy of the internal thread. Particularly in the case of a tap including a hard film such as titanium nitride formed on the surface of the body forming the tap for improved durability, long continuous chips tend to form which tend to get stuck.
In order to prevent chips from getting stuck, Patent document 1 proposes a tap in which a hard film formed on the surface of the body is removed from the rake faces to expose the body so that chips contact the surface of the body. This prevents the formation of long continuous chips, and thus prevents chips from getting stuck.
This tap is manufactured by forming rake faces and relief surfaces on the body, forming a hard film on the surface of the body, and removing the hard film from the rake faces. The hard film is removed by separately grinding the rake faces of the individual taps. This increases the manufacturing cost of the tap.
[Patent document 1] JP Patent Publication 2004-174698